(a) Field of the Invention
The invention relates to an image display apparatus having pixels where the brightness of each pixel is controlled by a display signal. More specifically, the invention relates to an image display apparatus having pixels where a light emitting element such as an organic EL (electro-luminescence) element is provided with each pixel and the brightness of the light emitting element is controlled by the amount of current flowing in the light emitting element. Moreover, the invention relates to an image display apparatus of an active matrix type where current supplied to a light emitting element is controlled by an active device, such as an insulating gate type FET (field effect transistor), which is provided in each pixel.
(b) Description of the Related Art
In general, a plurality of pixels are arranged in a matrix in an active matrix type image display apparatus, and an image is displayed by controlling the light intensity of each pixel in accordance with the brightness information provided from an external graphic source.
An organic EL image display apparatus is a self-emitting display by having a plurality of pixels and each of pixels having a light emitting element such as an OLED (organic light emitting diode). Organic EL image display apparatus are advantageous in comparison with a liquid crystal display because organic EL image display apparatus have favorable visibility, do not require a backlight, and they have a high display response speed. Brightness of each light emitting element is controlled by controlling the amount of current flowing in each light emitting element. In other words, the organic EL image display apparatus has a different property from the liquid crystal display in that the brightness of each light emitting element in the organic EL image display apparatus is directly controlled by the current flowing in each light emitting element, while the brightness of each pixel in the liquid crystal display is indirectly controlled by a light emitting means such as a backlight.
There are two general driving methods for driving organic EL image display apparatus. There is a passive matrix driving method and an active matrix driving method. The passive matrix driving method enables a simple panel structure in a display apparatus, but has difficulty realizing a large panel size and high definition of a display apparatus. Thus, a display apparatus using the active matrix driving method has been developed. In an organic EL image display apparatus using the active matrix driving method, current flowing in a light emitting element provided at each pixel is controlled by an active device provided at each pixel. The active device may be, for example, a thin film transistor which is a kind of an insulating gate type field effect transistor.
In a conventional organic EL image display apparatus configured as above, display operation is performed by a driving method where fixed gray levels regardless of brightness distribution by inputted RGB image data are used for display operation. That is, display operation is performed with fixed gray levels, and the display operation does not depend on whether the brightness of the display screen is high or low. The brightness of the display screen may be determined by the brightness distribution of RGB data. According to the above driving method, however, the brightness difference in a display screen becomes large when a difference between the number of ON-pixels and OFF-pixels is large. The brightness difference causes an uneven display in a screen. To solve the above problem, it is possible to reduce the steps of neighboring gray levels. However, this solution still has a problem of a representation limit due to a decrease of steps in neighboring gray levels. Thus, it is required that the steps of the neighboring gray levels should be variably adjusted in accordance with the brightness level of the display screen.